1. Field of the Invention
The subject invention relates to a method of forming high resilience slabstock polyurethane foam having random cell structures and displaying physical properties superior to latex foam while exhibiting latex-like feel and characteristics.
2. Description of the Related Art
The use of latex foam in high price, high quality mattresses has been common in the United States for several years, although market share has remained relatively low. Latex foam maintains a much higher share of the mattress materials market in Europe, because latex foam is considered to be a superior product with respect to its comfort and durability properties. Recently, U.S. consumers have been developing a higher sensitivity to sleep habits and a direct association between bedding quality and the quality of sleep. The mattress industry is focused on creating more specialized, higher price, higher quality products resulting in the growth of the latex foam share of mattress cushion materials. Traditionally, latex foam has been shown to display superior resilience, support factor, dynamic and static fatigue resistance when compared to polyurethane foam for cushioning applications. Additionally, latex foam is often advertised to provide superior pressure relief particularly in bedding applications. However, these comparisons have also typically been made between different densities of foam, i.e., 4.2 pcf density Latex foam vs. 2.5 pcf density HR polyurethane foam, while each has a hardness 28 lb. IFD at 25% deflection. Polyurethane foam is not typically formulated in the same density ranges as latex foam for cushion applications with the intention of meeting the significantly marketable differences between urethane and latex. The industry is also focused on finding other materials that perform and feel like these latex foams, such as high resilience polyurethane foams.
High resilience polyurethane foams are produced by reacting an isocyanate with an isocyanate-reactive component containing two or more reactive sites, generally in the presence of blowing agent(s), catalysts, surfactants and other auxiliary additives. The isocyanate-reactive components are typically polyols, polyesters, primary and secondary polyamines, or water. The catalysts used during the preparation of slabstock polyurethane foam promote two major reactions among the reactants, gelling and blowing. These reactions must proceed simultaneously and at a competitively balanced rate during the process in order to yield slabstock polyurethane foam with desired physical characteristics. Flexible slabstock foams are generally open-celled materials, which may require additional processing, such as crushing, to reach a desired openness.
Slabstock foam is produced in a foam machine that mixes the individual reactants, i.e., isocyanate, isocyanate-reactive components, and additives, in a continuous manner through a mix head and deposits the reaction product into a trough. The product begins to froth and rise out of the trough and overflows onto fall plates. On the fall plates, the product continues to rise and contacts a conveyor. The product cures as the conveyor carries it along a length forming the slabstock polyurethane foam. The conveyors are typically lined with a paper or plastic liner to allow for easy removal of the slabstock foam. As the foam exits the machine, it is cut into large blocks.
Various related art patents disclose methods of forming slabstock polyurethane foams. These methods include using blowing agents such as water, air, nitrogen, or carbon dioxide, as shown in U.S. Pat. No. 5,403,088. Typically, carbon dioxide liquid is added directly to the polyol component; however, it is also known in the art that it can be added to either or both components. The polyol component supply must be pressurized to maintain the carbon dioxide in the liquid state. As the product exits the mix head and as it froths and rises, the carbon dioxide changes states from a liquid to a gas and acts as a blowing agent. One primary reason for adding the carbon dioxide in a liquid state is to ensure that there is a sufficient amount of blowing agent to produce the foam having a desired density. However, one disadvantage of using liquid carbon dioxide is that the polyol component supply must be under pressure, which is expensive and can be dangerous to maintain the high pressures.
Yet another method, shown in U.S. Pat. No. 5,360,831, discloses adding carbon dioxide gas as a nucleation gas into either one of the polyol component or the isocyanate component streams for a foam-in-fabric process. The carbon dioxide gas thickens and increases the viscosity of the foaming mass to prevent the reacting components from entering the fine pores of the foam and fabric capsule, which allows these encapsulating materials to remain as is, functional, not compromised. Most importantly, the nucleating gas is thickening the froth to prevent penetration of the foaming mass into the pores of existing foam and fabric, therefore allowing the containing material to remain intact. Foam-in-fabric processes are different from slabstock foam processes in that the foam-in-fabric process is prepared in a batch process and makes only enough foam to fill a mold, whereas the slabstock process involves continuous reacting of the components. Fabric is positioned within the mold, and the components are mixed together and poured into the fabric. The components react, forming a foam that fills the fabric and forms the final product.
The use of other blowing agents, such as nitrogen gas or various other gases, is shown in WO 02/10245. One distinguishing factor between a blowing agent and a nucleation gas is the amount used and the effect that the blowing agent has on the slabstock foam. Typically, when a gas is added as a blowing agent, a large amount of the blowing agent is needed to expand the foam during the frothing and rising stages to control the density of the slabstock foam. The addition of more blowing agents results in a lower density foam.
On the other hand, the related art has used only a single nucleation gas to improve the characteristics of the foam. The nucleation gas, such as nitrogen, carbon dioxide, promotes irregular cell structure and reduces processing of the foam, such as crushing, after it has cured. If too much nitrogen gas is added as the nucleation gas, the cells in the slabstock foam are irregular shaped and become too large forming voids or pits in the slabstock foam. If the slabstock foam has too many voids, the slabstock foam loses its resilience and value. If too much carbon dioxide gas is added as the nucleation gas, the cells are too uniform and too dense. The slabstock foam prepared with too much carbon dioxide gas does not have similar physical properties, or performance characteristics, as that of latex foam. The slabstock foam of the related art in using a single nucleation gas has not yet achieved the latex-like feel, while also achieving the performance characteristics associated with the latex foam.
However, to date, the industry has been unable to produce a slabstock polyurethane foam having performance characteristics of latex foam while also having a substantially similar feel to that of the latex foam. Further, the industry has been unable to produce a slabstock polyurethane foam that is substantially free of toluene diisocyanate (TDI) and formed solely from diphenylmethane diisocyanate (MDI). The slabstock polyurethane foams produced from TDI do not have as improved performance characteristics as slabstock polyurethane foams produced from MDI. U.S. Pat. Nos. 6,068,790 and 5,480,916 and Patent Cooperation Treaty publication WO 01/32735 disclose using mixtures of TDI and MDI. However, they do not disclose using solely MDI as the isocyanate component to produce the slabstock polyurethane foam having latex-like feel and performance characteristics. Great Britain Patent No. 2,093,852 discloses using an isocyanate component having a mixture of MDI and polymethylene polyphenyl polyisocyanates.
Patent Cooperation Treaty publication WO 95/09886 discloses preparing a high resilience foam using an isocyanate component including a mixture of MDI. In each of these references, the foam produced does not have latex-like feel and performance characteristics. Specifically, the ""886 publication uses a mixture of 75 parts by weight of 4,4xe2x80x2-MDI and 25 parts by weight of polymeric MDI based on 100 parts by weight of the isocyanate component. The resulting foam has a density of less than 2.9 pcf, a tensile strength of less than 7 psi, and a tear resistance of less than 0.4 ppi. The ""886 patent does not disclose forming a polyurethane foam with a density greater than 3 pcf. It would be advantageous to form a polyurethane foam having density greater than 3 pcf that has latex-like feel and performance characteristics.
The industry has been able to achieve polyurethane foam having performance characteristics similar to that of latex foam using TDI and mixtures of TDI and MDI, but the density of the slabstock foam is less than that of the latex foam and it does not have the similar feel of latex foam. The industry has also been able to make a polyurethane foam having a low density solely from MDI, but the foam does not have the required latex-like feel and performance characteristics. Accordingly, it would be advantageous to provide a method of forming a high resilience slabstock polyurethane foam having random cell structures yielding a latex-like feel and performance characteristics, such as a indentation force deflection (IFD) equal to that of latex foam.
The subject invention provides a method of forming high resilience (HR) slabstock polyurethane foam having random cell structures to produce latex-like feel and characteristics. The method includes the steps of providing an isocyanate-reactive component and providing an isocyanate component substantially free of toluene diisocyanate comprising diphenylmethane-2,4xe2x80x2-diisocyanate present in an amount of from 15 parts by weight to 55 parts by weight based on 100 parts by weight of the isocyanate component and diphenylmethane-4,4xe2x80x2-diisocyanate present in an amount from 45 parts by weight to 85 parts by weight based on 100 parts by weight of the isocyanate component to react with the isocyanate-reactive component. A first nucleation gas is provided under low pressure and is added into at least one of the isocyanate-reactive component and the isocyanate component to produce a first cell structure in the foam. A second nucleation gas, different from the first nucleation gas, is provided under low pressure and is added into at least one of the isocyanate-reactive component and the isocyanate component to produce a second cell structure in the foam that is different from the first cell structure such that the first cell structure and the second cell structure enhance the latex-like feel and characteristics of the slabstock polyurethane foam.
Accordingly, the subject invention provides a HR slabstock polyurethane foam formed from an isocyanate component substantially free of toluene diisocyanate solely from diphenylmethane diisocyanate. The resulting foam has a random cell structure that provides a latex-like feel and performance characteristics. While a combination of two nucleation gases yields the latex-like feel and characteristics, the use of diphenylmethane diisocyanate blends as described, produces the slabstock polyurethane foams having superior physical properties to traditional latex foam. These slabstock polyurethane foams display superior durability by having improved tensile strength, elongation, tear resistance, dynamic and static fatigue resistance compared with latex foam at similar densities. More importantly, these polyurethane foams are in compliance with California Technical Bulletin 117 test (Cal 117), while latex foam is not. Cal-117 compliance is one of the requirements in California or as specified for foams used in residential furnishing. In addition, with the use of additional flame retardant like melamine, these polyurethane foams also meet British Standard 5852:Crib 5 requirement. The HR slabstock polyurethane foam is capable of use in any cushioning application that has traditionally been manufactured with latex foam. The HR slabstock polyurethane foam outperforms similar polyurethane foams formed from TDI and TDI/MDI mixtures and similar latex foams having a substantially similar density and hardness.